Nitrate continually accumulates in most RAS and has been shown to reduce production in high concentrations, thereby limiting long-term water reuse. Denitrification, a process in which facultative anaerobic microbes reduce nitrate to harmless dinitrogen gas, may allow for greater water reuse. Many denitrification filters are media based and may be unsuitable for systems with high solids concentrations such as biofloc-style techniques. Reactors relying on heterotrophic microbes also require organic carbon additions to fuel microbial activity, which increases costs for producers. In addition, some media types can be expensive. Low- cost alternatives such as wood chips for media and utilizing endogenous solid waste for carbon may lower costs while maintaining high rates of denitrification. The purpose of this study was to evaluate the potential for using no media versus woodchips in in-line denitrification bioreactors as well as to evaluate carbon sources, including fermented sludge supernatant, ethanol, and no added carbon.
The study consisted of six treatments with four replicates each and lasted 90 days. Each system included a 1 m3 shrimp tank stocked at 250 shrimp m-3. Water from each shrimp tank was pumped through a 3 L foam fractionator, an 18 L settling chamber, an 18 L moving bed biofilter, and a 28.7 L denitrification column. Sludge was collected from the settling chambers and allowed to ferment for a week prior to additions. Ethanol was added to maintain a 3:1 C:N ratio based on the daily nitrogen additions through feed. The concentrations of TAN, nitrite, nitrate, phosphate, alkalinity, turbidity, and TSS/VSS were measured weekly. Temperature, DO, pH, salinity and ORP were each measured twice daily.
Results indicate that nitrate was significantly lower in the WC treatments than in the NM treatments and in the ethanol treatments than in the no-C and sludge treatment. Shrimp survival and average weight were greater in the no media treatment than in the woodchip treatment. The ethanol treatment had greater average weight but lower survival than both the no added carbon and the sludge treatments. This experiment indicates that there can be a range of implications based on media and carbon types in denitrification reactors. Some factors seem to benefit water quality, but may have negative effects on shrimp. Such considerations are important for producers who wish to balance nitrate remediation while optimizing shrimp production.